It is proposed to evaluate permeability of electrotonic junctions between embryonic cells. Use of ion sensitive microelectrodes should allow accurate correlation of junctional resistance with permeability to appropriate ions injected iontophoretically. Permeability to other small molecules will be tested by iontophoretic or pressure injection. Fluorescence, histochemical staining or isolation of coupled uninjected cells will be used to assay transjunctional movement. Preparations suitable for determining permeability in adult tissues will be developed, and comparisons made with embryonic cells. The role of cell movement in junction formation will be assayed by use of specific inhibitors. Existence of precursor molecules for junction formation will be studied by determining the cell's ability to form junctions in the presence of inhibitors of protein synthesis. The mobility and segregation of potential sites of coupling in the membrane will be investigated by marking specific regions of the cell surface. The goal is to determine the structure of the hypothesized channels connecting the cell cytoplasms and to analyze the mechanisms involved in their formation. This information should be uniquely accessible in the experimental systems to be employed and it should provide insight into the role of these junctions in development as well as in functioning of the adult tissues in which they are so prevalent.